What accounts for the rapid gain in overall brain size after birth?

The correct answer, myelinization, refers to the process where a fatty substance called myelin forms around the axons of neurons. This process is crucial for the efficient transmission of electrical signals within the brain and between the brain and other parts of the body. After birth, myelinization significantly contributes to the rapid gain in overall brain size, as it enhances neural connectivity and speeds up communication between neurons. As myelin sheaths develop, the brain becomes more efficient in its functioning, which is essential for cognitive and motor development in infants and toddlers.

Other processes like neurogenesis (the formation of new neurons) and synaptogenesis (the formation of synapses between neurons) also play vital roles in brain development but do not account for the overall increase in brain size in the same way that myelinization does. Brain plasticity refers to the brain's ability to adapt and change in response to experiences, which is important for learning and development, but again does not directly explain the increase in brain size following birth.